NAME

vkbd — the virtual AT keyboard interface

SYNOPSIS

devicevkbd

DESCRIPTION

The vkbd interface is a software loopback mechanism that can be loosely described as the
virtual AT keyboard analog of the pty(4), that is, vkbd does for virtual AT keyboards what
the pty(4) driver does for terminals.
The vkbd driver, like the pty(4) driver, provides two interfaces: a keyboard interface like
the usual facility it is simulating (a virtual AT keyboard in the case of vkbd, or a
terminal for pty(4)), and a character-special device “control” interface.
The virtual AT keyboards are named vkbd0, vkbd1, etc., one for each control device that has
been opened.
The vkbd interface permits opens on the special control device /dev/vkbdctl. When this
device is opened, vkbd will return a handle for the lowest unused vkbdctl device (use
devname(3) to determine which).
Each virtual AT keyboard supports the usual keyboard interface ioctl(2)s, and thus can be
used with kbdcontrol(1) like any other keyboard. The control device supports exactly the
same ioctl(2)s as the virtual AT keyboard device. Writing AT scan codes to the control
device generates an input on the virtual AT keyboard, as if the (non-existent) hardware had
just received it.
The virtual AT keyboard control device, normally /dev/vkbdctl⟨N⟩, is exclusive-open (it
cannot be opened if it is already open) and is restricted to the super-user. A read(2) call
will return the virtual AT keyboard status structure (defined in <dev/vkbd/vkbd_var.h>) if
one is available; if not, it will either block until one is or return EWOULDBLOCK, depending
on whether non-blocking I/O has been enabled.
A write(2) call passes AT scan codes to be “received” from the virtual AT keyboard. Each AT
scan code must be passed as unsignedint. Although AT scan codes must be passes as unsignedints, the size of the buffer passed to write(2) still should be in bytes, i.e.,
static unsigned int codes[] =
{
/* Make Break */
0x1e, 0x9e
};
int
main(void)
{
int fd, len;
fd = open("/dev/vkbdctl0", O_RDWR);
if (fd < 0)
err(1, "open");
/* Note sizeof(codes) - not 2! */
len = write(fd, codes, sizeof(codes));
if (len < 0)
err(1, "write");
close(fd);
return (0);
}
Write will block if there is not enough space in the input queue.
The control device also supports select(2) for read and write.
On the last close of the control device, the virtual AT keyboard is removed. All queued
scan codes are thrown away.

SEEALSO

HISTORY

AUTHORS

CAVEATS

The vkbd interface is a software loopback mechanism, and, thus ddb(4) will not work with it.
Current implementation of the syscons(4) driver can accept input from only one keyboard,
even if it is virtual. Thus it is not possible to have both wired and virtual keyboard to
be active at the same time. It is, however, in principal possible to obtain AT scan codes
from the different sources and write them into the same virtual keyboard. The virtual
keyboard state synchronization is the user's responsibility.